In French Patents 1 438 330, 2 105 710 and 2 146 612, there has already been described the production of a watertight and thermally insulating tank, built into the bearing structure of a ship and made up by two successive watertightness barriers, one being a primary one, in contact with the liquefied gas transported, and the other a secondary one, located between the primary barrier and the bearing structure of the ship, these two watertightness barriers being alternated with two layers of thermal insulation termed "insulating barriers". In these embodiments, the primary and secondary insulating barriers are made up of parallelepipedal compartments filled with a particular insulant and the primary and secondary watertightness barriers are made up of metal strakes, for example made of invar, with turned-up edges welded on either side of a weld flange.
In French Patent 2 504 882, an embodiment of this type of tank is proposed in which the secondary insulating barrier is made up of parallelepipedal compartments filled with insulant and the primary insulating barrier is made up of plates formed of a cellular layer mounted on a rigid panel. The rigidity of the plates of the primary insulating barrier allows better resistance with regard to impacts produced on the walls of the tank by the movements of the liquid during transportation, which movements are due to the rolling and pitching of the ship. Unfortunately, this device exhibits a defect: what happens is that the primary barrier is fastened directly onto the bearing structure of the ship by anchoring members, which pass through the secondary watertightness barrier. Now, it has become apparent that this technique is liable to generate stress concentration zones under certain conditions, which is unfavourable from the safety point of view; what is more, the anchoring members establish a direct thermal bridge between the primary barrier and the bearing structure of the ship, which is unfavourable from the insulation performance point of view.
In French Patent Application 2 683 786, a tank of the type defined hereinabove is described, in which the secondary insulating barrier is made up of a set of heat-insulated compartments, each compartment including, in line with each groove intended for the fitting of a fastening means ensuring that the primary insulating barrier is pressed elastically on the secondary insulating barrier, a thick internal bulkhead fixed to the faces which delimit the compartment, the retaining members employed for holding the secondary insulating barrier on the bearing structure of the ship being, away from the corners of the tank, aligned in line with the grooves in which the fastening means are inserted. According to a technique described previously by the applicant company, the connection corner of the tank walls in the zones where the transverse bulkheads of the ship meet the double hull, was produced in the form of a ring, the structure of which remains constant along the entire length of the ridge of intersection of the said transverse bulkhead with the double hull of the ship. In order to produce a corner formed by the double-hull of the ship and a transverse bulkhead, it was proposed, in this French Patent Application 2 683 786, to fasten two perpendicular anchoring bands joined by L-shaped angle brackets to the two secondary watertightness barriers onto the perpendicular bearing walls by means of a one-way joining piece, the said L-shaped angle brackets being joined together by a joining band perpendicular to the plane bisecting the corner in question, at least one of the two anchoring bands being extended, substantially in its plane, beyond the L-shaped angle bracket which is joined to it, in order to meet the primary leaktightness barrier associated with one of the bearing walls of the corner in question, the primary leaktightness barrier associated with the other bearing wall being connected, by watertight welding of a bracket-shaped strip, to its abovementioned counterpart and, possibly, to the anchoring band situated in its plane.
This embodiment of the tank corners gives satisfaction but corresponds to a relatively high cost price. Furthermore, it is known that when the ship is moving in heavy seas, the deformation of the girder which it constitutes generates very significant tensile stresses at the primary and secondary watertightness barriers which stresses, in fact, add to the tensile stresses generated in these watertightness barriers when the tank is subjected to cold. In the device described in French Patent Application 2 683 786, the take up of these tensile loads is effected substantially in the plane of the primary leaktightness barriers, that is to say at a distance from the intersection ridge of the bearing structure which is equal to the thickness of the primary and secondary insulating barriers together. Although this arrangement does not pose any problems as regards the double hull of the ship it does, in contrast, require that part of the double transverse bulkheads which is close to the intersection ridge to be reinforced. The cost of carrying out this reinforcement adds to the cost due to the complexity of the connecting ring proposed by this French Patent Application 2 683 786 and generates an increase in the cost price of the ship.